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A Flood Warning and Evacuation Plan is a document that set out what "site users" will do in the event of flood, it is primarily concerned with their safety. "Site Users" can be residents of a building or workers & customers at a shop or other venue. A Flood Warning and Evacuation Plan is normally required where Flooding could be a danger to site users.
The are many dangerous things that can occur when a flood occurs these include (but not limited to) the following conditions:
Water is heavy. Have you ever tried to wade across a shallow stream? Even for a young fit person fast flowing water that is more than 20cm deep can become problematic. If we consider infirm persons, and deeper water than evacuating a flood area is not always practical.
In any Flood Warning and Evacuation Plan it will be necessary to establish what these hazards are. Will the water be fast flowing? In order to establish this we will look at available data and try to assess the risk.
In the UK flood water is typically cold. People who are trapped in buildings will need a safe refuge that is above the flood level, so they can stay warm and dry. The safe refuge will be need to be provisioned accordingly.
In most urban flooding events, sewer water and flood water freely mix, and one can expect flood water as a consequence to have very high coliform (gut bacteria) counts. If site users are likely to take refuge they will need a clean supply of drinking water . . . .
. . . . otherwise they will become dehydrated. In our experience the longest period of refuge to be expected is around 5 days. So you can imagine you will need a water source for a period of this duration, this duration is based on historic flood events (Thames Estuary) where tidal flood water was trapped behind overtopped flood defenses.
If site users leave it too late to evacuate, they can often be injured in the process. Flood water is often very murky, and so when wading through it you can not see what lies beneath. Unfortunately during flood drain covers, and manhole cover are often dislodged and they are a nasty hazard to encounter when walking. Floating debris is also a problem.
To avoid some of the above hazards it may be advisable to include a safe refuge (there is an interesting picture on this page) in your proposed development. For a single dwelling this can be any room above the flood water. . .1st floor is typically OK . . . although in some areas even the first floors would be submerged. But for a block of flat for example a designated room may need to be provided for people to take refuge.
The room should be well provisioned based on the number of people expected to occupy it, and should have provisions for sanitation and perhaps limited entertainment!
Of course in a perfect world the best thing to do would be to evacuate the area before it becomes flooded. Luckily the Environment Agency offer a text message subscription service, whereby (hopefully) you will receive a text message ahead of a flood event.
Of course there are so many warning about weather, and flooding in the modern world it is hard to pick out the ones which are actually inform of real dangers. But for those who pay attention these warning may prove valuable.
The Flood Warning and Evacuation Plan will typically detail a safe route where by a person and group of persons, may find their way to safety. It will include a route marked with postcodes and navigational references so that they might leave the area by driving of walking, dependent on the time they will have.
Some large low lying areas will flood very quickly and over massive distances (Estuarine Areas: Thames - London or Avon - Avonmouth) so evacuation on foot may not be practical, you may not escape in time.
It is all well and good to write about these things in a report, submitted with planning applications, but to our knowledge there is no soft landing scheme associated with these reports. As such we expect most stay on the planning portal, or in a drawer somewhere unread, and as such have very little value. If site users were told about them, then they may prove useful, and that certainly should be the case. But until that happens they are a badly implemented piece of red tape.